Obsessive-compulsive disorder (OCD) is a severe, heritable condition with a lifetime prevalence of about two percent of the population. The mode of inheritance is not well understood but is likely complex, involving multiple loci of small to moderate effect. Our laboratory has been active in studies of OCD and of its genetics for over 15 years, and in 2001 became one of the founding sites of a multi-center genetic study of OCD (P.I.: Dr. Gerald Nestadt of Johns Hopkins University). This study was approved via a competitive NIMH extramural grant application. Due to the accumulation of evidence supportive of genetic contributions to OCD, a series of association studies of candidate genes has been undertaken and reported in the literature, but only one, very small genome- wide scan of OCD had previously been reported. Our OCD genetic studies in the NIMH IRP contribute DNA and family evaluation data to this national multi-site, genome-wide study of OCD which described its methodology (Samuels et al., 2006) and reported the first results from a 10cM genome scan this year (Shugart et al., 2006). ?Suggestive linkage signals were revealed by multipoint analysis on chromosomes 3q27-28 (P=0.0003), 6q (P=0.003), 7p (P=0.001), 1q (P=0.003), and 15q (P=0.006). Using the 'broad' OCD definition, the strongest evidence for linkage was found on chromosome 3q27-28. The maximum overall Kong and Cox LODall score (2.67) occurred at D3S1262 and D3S2398, and simulation based P-values for these two signals were 0.0003 and 0.0004, respectively, although for both signals, the simulation-based genome-wide significance levels were 0.055. Covariate-linkage analyses implicated a possible role of gene(s) on chromosome 1 in increasing the risk for an earlier onset form of OCD. The collaborative group is currently pursuing fine mapping in the five regions giving suggestive signals, with a particular focus on 3q27-28. Given probable etiologic heterogeneity in OCD, mapping gene(s) involved in the disorder may be enhanced by replication studies, large-scale family-based linkage studies, and the application of novel statistical methods?. [unreadable] [unreadable] In addition, within the NIMH-IRP, exploratory analyses of DNA, clinical features and personality and other characteristics of ~300 OCD probands and of disorders related to OCD are being used to assess the candidacy status of gene variants and to better define the familial OCD phenotype. [unreadable] [unreadable] In one such study, factor and cluster analysis of 70 OCD symptoms rated in each of the 317 patients with OCD from our Lab revealed a four factor grouping of symptoms which showed specific relationships to comorbid psychiatric disorders (Hasler et al., 2005). Thus, Factor I (aggressive, sexual, religious and somatic obsessions, and checking compulsions) was broadly associated with comorbid anxiety disorder and depression; Factor II (obsessions of symmetry, and repeating, counting and ordering/arranging compulsions) with bipolar disorders and panic disorder/agoraphobia; and Factor III (contamination obsessions and cleaning compulsions) with eating disorders. Factors I and II were associated with early onset OCD. The frequent co-occurrence of OCD with other psychiatric disorders and the relatively specific association patterns between OCD symptom dimensions and comorbid disorders support the importance of OCD subtyping for genetic, treatment, and other research studies of this heterogeneous disorder. In a gene-based follow-up to this study, an association was replicated between the SERT 5HTTLPR functional polymorphism and the symmetry/counting/ordering (Factor II) dimension of OCD symptoms (Hasler et al., 2006). In further studies of the SERT gene in OCD and related disorders such as Tourette?s syndrome, the 5HTTLPR together with a newly-discovered SNP (rs25531) within it were found to be associated with OCD in both a case-control and a trios study (Hu et al., 2006) and improved methods to genotype these and other variants in OCD were reported (Wendland et al., 2006a; 2006b.)